1. Field of the Invention
The invention relates to a drive system for a motor vehicle.
2. Description of the Related Art
The problems underlying the invention apply to any vehicle but are explained in detail with reference to a passenger car.
A hybrid vehicle generally refers to a vehicle with a drive system that has a number of drive units, such as a combustion engine and an electric motor. Parallel hybrid drives generally are used in hybrid vehicles to produce as high an energy efficiency as possible by allowing the electric motor and the combustion engine to introduce torque into a transmission either as alternatives or cumulatively. Moreover, it should also be possible to use the electric motor as a generator. For example, braking energy of the vehicle is a form of kinetic energy, and desirably should be used to charge electrical energy storage devices. Frequent starting and acceleration processes of the kind that generally occur in urban traffic, for example, preferably are carried out or assisted by the electric motor in the case of a hybrid motor vehicle since operating the combustion engine with frequent changes in load results in increased fuel consumption and pollutant emissions. In contrast to combustion engines, electric motors have a high torque virtually from a standstill, even at low motor speeds, and the electric motor therefore almost is predestined for starting and acceleration processes. A combustion engine, on the other hand, can only be operated with a high efficiency at its rated speed, e.g. when the vehicle is driven at a constant high speed. To enable the advantages of the combustion engine to be combined with the advantages of the electric motor, it is therefore necessary to design the drive system in such a way that both the power of the combustion engine and the power of the electric motor can be coupled into the drive system.
A drive system for a motor vehicle with a shift transmission, a combustion engine and an electric machine is described in U.S. Pat. No. 6,506,139 B2. The combustion engine can be connected nonpositively to the input shaft of the transmission via a friction clutch to drive an output shaft of the transmission. The electric machine can be connected to the output shaft or the input shaft via further friction clutches or positive clutches. In this arrangement, the electric machine is used to start the combustion engine, to generate electrical energy and essentially to smooth out interruptions in tractive effort that occur as the transmission is shifted due to the necessary decoupling of the combustion engine from the drive train.
It is therefore the underlying object of the present invention to provide an improved drive system.